Drug dealers are becoming so frustrated by seizures of cocaine that they are selling it at only 5 per cent purity after mixing in cutting agents such as boric acid and worming powder, according to the head of the Serious Organised Crime Agency (Soca).

Sir Stephen Lander said that the proportion of pure cocaine found by officers in some seizures was small compared with the average dilution. Typically, packages of the class A drug sold to British users are 20 per cent pure.

He told the Commons Home Affairs Select Committee that operations to intercept shipments around the world, together with increased international co-operation in disrupting markets, meant that pure cocaine had never been more expensive in Britain.

Sir Stephen said that a third of the estimated 860 tonnes of cocaine produced by cartels was seized by enforcement agencies around the world. Soca officers have played a key role in training police in Sierra Leone, who seized a shipment from South America last year. Two Colombians arrested in the seizure are serving a sentence of 999 years in the United States.

“That is what we have to do to protect this country,” Sir Stephen told MPs. “Waiting at the border is leaving it too late.” He admitted that, despite a large deployment in Kabul, his officers were having less of an impact on the heroin trade. Major problems included difficulties in international relations and a lack of co-operation from Iran and Russia — both key transit countries for heroin trafficking.

Sir Stephen, who steps down as chairman of Soca this month, said that he did not know who his successor would be, but he said the organisation, which has been criticised for having poor intelligence and top-heavy management, was “on the right track”.

He said: “We certainly had some teething problems but we’ve got the right people in our sights now. We’ve come a long way in three years.”

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